In the aforementioned co-pending application there is disclosed an amplifier responsive to a signal source comprising at least several stages activated only to one of two bi-level states (sourced and unsourced) controlled in response to the value of a signal source exceeding a different threshold for each stage. The thresholds and source are arranged so that the stages change between the bi-level states as the value of the source changes relative to the thresholds of the different stages. Bi-level outputs associated with the bi-level states of the plural stages are summed together. The values of the thresholds are continuously varied by a small amount relative to a predetermined maximum value for the source. In the preferred embodiment, the threshold values are varied by applying a wave having a continuously varying amplitude in parallel to the various stages. Preferably, the continuously varying wave is derived from a variable frequency voltage controlled oscillator and is a triangular wave having a frequency determined by the amplitude and frequency of the signal source. As the amplitude and frequency of the signal source increase the triangular wave frequency increases from a nominal value thereof. The amplifier disclosed in the co-pending application is, in a preferred use, responsive to an audio signal source, e.g. speech or music, and is part of an amplifier for controlling plate modulation of a high-power amplitude modulated radio frequency transmitter. The output wave derived from the amplifier is a high-voltage replica of the input signal supplied to the amplifier.
In the preferred embodiment of the prior art amplifier, 48 output stages are employed. The output stages are powered by an AC to DC converter arrangement driven by a three-phase source. The AC to DC converter arrangement includes two primary windings and a secondary winding for each output stage. Each secondary winding is connected to a separate rectifier for supplying DC energizing voltage to the stage associated with it. One primary winding is coupled via a magnetic core to the secondary windings powering output stages 1-24, while the second primary winding is coupled via a second core to the secondary windings which drive output stages 25-48. The output stages are energized by control circuitry for them so that consecutively numbered stages are activated to a first of the bi-level states and the remaining output stages are activated to the other bi-level state. In the preferred embodiment of the prior art, there is a continual change at a predetermined frequency of which output stages are sourced. The change occurs even if there is no change in the amplitude of the input signal over a certain time. Such continuous changing obviates the tendency for certain stages to be virtually continuously dissipating power while other stages virtually never dissipate power.
The amplifier disclosed in the above-mentioned application functions admirably for many situations. For other situations, the amplifier, through actual testing, has been found to produce excessive noise components. I have determined that two sources of the excessive noise components are (1) unequal loading of the two primary windings coupled to the secondary windings which drive the output stages and (2) excessively high amplitude components associated with changing the thresholds in switching the stages between the bi-level states.
The first noise components are at a frequency determined by the number of output stages and the frequency at which the stages are switched. To understand how the first noise components are derived, consider the situation wherein the input signal is constant, at zero AC amplitude, resulting in one-half of the output stages being in the sourced state and the remaining output stages being unsourced. At one instant of time, stages 1-24 are sourced, while stages 25-48 are unsourced. At a second time instant, a few milliseconds after the first time instant, stages 1-12 and 37-48 are unsourced simultaneously with stages 13-36 being sourced. At a third time instant, displaced a few milliseconds from the second time instant, stages 1-24 are unsourced and stages 25-48 are sourced.
During the first mentioned time instant, the first primary winding is fully loaded and the second primary winding is completely unloaded. Thereby, the voltage supplied by the first transformer to stages 1-24 is a relatively low voltage. The sum of the output voltages derived from stages 1-24 is thereby somewhat lower than expected. At the second time instant, the voltages of the two primary windings are approximately halfway between the minimum voltage, as occurs when the primary windings are fully loaded, and the maximum transformer primary windings, which occurs when the primary windings are completely unloaded. Thereby, the voltages supplied to stages 13-36 are greater than during the first time instant and the sum of the output voltages derived from these 24 stages during the second instant is somewhat greater than the sum of the output voltages derived from stages 1-24 during the first-mentioned instant. During the third instant, the second primary winding is fully loaded, causing the sum of the output voltages of stages 25-48 to be virtually the same as during the first mentioned instant. Hence, the resultant sum of the output voltages of all 48 stages varies at a frequency determined by the number of stages and the switching frequency of the stages, even though there is no change in the amplitude of the input signal driving the amplifier.
The second source of the noise components, excessively high amplitude components associated with changing the thresholds of the states in switching the stages between the bi-level states, occurs as a result of the tendency of the variable frequency voltage controlled oscillator to produce sidebands in the frequency band of the input signal being amplified. The voltage controlled oscillator, preferably of the type disclosed in commonly assigned U.S. Pat. No. 4,896,372, has a nominal frequency, such as 70 kHz, considerably above the 5 or 10 kHz bandwidth of the input audio signal source. In response to increases in the amplitude and frequency of the input source, the output frequency of the voltage controlled oscillator increases relative to the nominal 70 kHz value thereof. In addition to the increased frequency of the voltage controlled oscillator output, I have found that the oscillator derives sidebands having frequency components that can overlap into the spectrum of the input signal. Due to the nature in which the sidebands are derived, as governed by Bessel function relationships, the amplitude of the overlapping sidebands can be relatively high. Since the voltage controlled oscillator output determines the threshold level of the output stages, the relatively high amplitude overlapping sidebands derived from the voltage controlled oscillator introduce noise in the output of the amplifier.
It is, accordingly, an object of the present invention to provide a new and improved amplifier having at least several output stages driven only between bi-level outputs which are summed together, wherein the noise content of the signal derived by the amplifier is significantly reduced relative to the prior art.
Another object of the present invention is to provide a new and improved amplifier including at least several output stages having only bi-level outputs tha are summed together, wherein the stages are driven by plural primary windings in such a manner that the primary windings are at all times approximately equally loaded.
A further object of the present invention is to provide a new and improved amplifier including at least several stages having only bi-level outputs that are summed together and wherein variable thresholds of the stages are controlled by an oscillator which derives a variable frequency output as a function of an input signal and tendencies for sidebands of the oscillator to be perceptively coupled to the output of the amplifier are appreciably reduced.